vc4: Fill thread switching delay slots

vc4: Fill thread switching delay slots

Scan for instructions without a signal set in front of the switching
instruction and move the signal up there.

shader-db results:

total instructions in shared programs: 94494 -> 93027 (-1.55%)
instructions in affected programs:     23545 -> 22078 (-6.23%)

v2: Fix re-emitting of the instruction in the loop trying to emit NOPs,
    drop a scheduling change from branch delay slots. (by anholt)

Signed-off-by: Jonas Pfeil <pfeiljonas@gmx.de>